SciencePrimer is a free web site that has some great science resources. The site has illustrations (animations with explanations) of concepts, practice questions and problems with hints and solutions, calculators and conversion tools, and a science glossary.
The illustrations are really good. They include interactive animations that let you change different specifications or characteristics, an explanation of the topic, and videos.
This site has some great resources that can help students understand and learn science resources.
MyPhysicsLab - Free Physics Simulations
PhET - excellent, free, virtual labs and simulations for science
Physion - free Physics Simulation Software
Great Physics Resources for Students and Teachers